a) Field of the Invention
The present invention relates to a semiconductor device manufacturing technique particularly suitable for removing a native oxide film from the surface of a silicon substrate. In this specification, the term "native oxide film" is used to mean imperfect oxide films other than thermal oxidation films.
b) Description of the Related Art
The surfaces of many semiconductors and metals are easily oxidized in air and native oxide films are formed thereon. A native oxide film formed on the surface of a Si substrate has a thickness of about 2 nm (measured by ellipsometry) and is called an imperfect silicon oxide film.
A silicon oxide film is an insulating material. If a silicon oxide film remains on the surface of a Si substrate, the contact resistance of an electrode connected to a conductive region on the Si substrate is increased.
A native oxide film on a Si substrate surface is a silicon oxide film having an imperfect crystallinity and having a film quality inferior to a thermally oxidized Si film. As the size of a MOSFET is scaled down, the gate oxide film often becomes as thin as 10 nm or less. For example, in a case where gate oxide film of 5 nm thickness is to be formed, if a poor quality native oxide film of 2 nm thickness is left on the surface of a Si substrate, a resultant gate oxide film will have degraded film characteristics.
A wet process using diluted hydrofluoric acid is known as a process of removing a native oxide film on a Si substrate (G. S. Higashi et al., Appl. Phys. Lett., 56, p. 656, 1990). A native oxide film on the surface of a Si substrate immersed in a diluted hydrofluoric acid solution dissolves to expose a bare Si surface, and hydrogen atoms terminate the dangling bonds at the Si substrate surface.
Although native oxide film removal using diluted hydrofluoric acid forms a stable Si substrate surface on the (1 1 1) plane, it forms a surface with a lower stability on the (1 0 0) plane. This wet process using diluted hydrofluoric acid presents some difficulties in proceeding directly to the next dry process because of a danger that the Si substrate surface may be again oxidized while the substrate is transported to the dry process system.
As another process of removing a native oxide film on a Si substrate, a dry process using hydrogen plasma is known (A. Kishimoto et al., Jpn. J. Appl. Phys., 29, p. 2273, 1990). This dry process of removing a native oxide film on a Si substrate can essentially and easily be followed by the next dry process. However, there is a danger that the Si substrate surface may be damaged because the Si substrate exposed in a plasma is impacted by collisions with high energy particles such as ions and electrons.
As still another process of removing a native oxide film on a Si substrate, a dry process using hydrogen atoms (radicals) is known (T. Takahagi et al., J. Appl. Phys., 68, p.2187, 1990). Although this dry process using hydrogen atoms (radicals) presents no danger of damage to the Si substrate surface, the process speed is low, and according to this paper, it takes time or the order of an hour to remove a native oxide film on a Si substrate surface.
As described above, it is not always easy to efficiently remove a native oxide film on a semiconductor substrate surface at any desired time during semiconductor device manufacturing processes.